The present disclosure is related to the field of organic and polymer electronics, more specifically it is related to the field of organic light-emitting devices.
In the last decade, remarkable progress has been made in synthesizing conjugated polymers, in understanding their properties and in developing them for use in electronic and optical devices. Currently, different types of devices can be made by using these materials, for example organic light-emitting diodes (OLEDs), organic photo detectors, organic photovoltaic cells and organic field-effect transistors.
Organic light-emitting devices are typically generated by sandwiching one or more appropriate organic layers between two conductive electrodes. This concept is proposed in U.S. Pat. No. 4,539,507. When an electric field is applied to the device, negatively charged carriers are injected into the organic layer or organic layers from one electrode, and positively charged carriers are injected into the organic layer or organic layers from the other electrode. Under influence of the applied field, the injected carriers travel through the organic layer or organic layers. When two oppositely charged carriers meet each other on one molecule or polymer segment, they recombine and emit light.
Another example of prior art organic light-emitting devices is the organic light emitter with improved carrier injection as disclosed in U.S. Pat. No. 6,720,572. This device comprises an organic light-emitting layer and an organic semiconductor layer that enhances injection, interposed between two contacts. This device can also be a transistor comprising three electrodes, a gate, a drain and a source. By applying a gate bias, charge carriers are accumulated at the interface between the semiconductor and the gate dielectric. If an additional drain bias is supplied, these carriers are injected into the light-emitting layer, where they can recombine and emit light.
The main disadvantage of the former light-emitting devices is that the light emission occurs very close to a metallic contact. When the light output is mainly perpendicular to the surface of the device, the presence of a metallic contact at a distance of 30 nm or more from the place of photon creation is acceptable. This is how a light-emitting diode is normally used. However, if it is the intention to guide photons in a waveguide, where the waveguide includes the physical layer in which the photons are generated, the close proximity of the metal contact introduces high optical absorption losses. In the state-of-the-art different solutions exist to circumvent this problem.
J. H. Schön et al. introduced the idea of an ambipolar light-emitting transistor, in “A Light-Emitting Field-Effect Transistor”, Science, 2000, 290 (3), 963-965. This device comprises three contacts, a gate dielectric and an ambipolar light-emitting semiconductor material. By applying an appropriate bias to the drain and the gate electrode, electrons and holes are injected from the source and the drain. As the ambipolar material can conduct both types of carriers, the holes and electrons recombine between the two contacts.
Another example of prior art organic light-emitting devices, having the light emission far away from the contacts, is the three-terminal light-emitting transistor reported by S. De Vusser et al., in “Light-emitting organic field-effect transistor using an organic heterostructure within the transistor channel”, Applied Physics Letters 89, 2006, 223504. The light-emitting devices comprise a heterojunction in the accumulation channel.
The disadvantages, however, of the transistor structures of J. H. Schön et al. and S. De Vusser et al. are threefold. In the first place, for a given maximum voltage bias, only relatively low current densities can be achieved, resulting in low intensities of the emitted light. Secondly, the devices are three-terminal devices. Finally, the device structure makes it difficult to incorporate an additional light-emitting layer.